CN113359367A

CN113359367A - Optical device and optical transceiver including optical modulator

Info

Publication number: CN113359367A
Application number: CN202110047772.0A
Authority: CN
Inventors: 杉山昌树
Original assignee: Fujitsu Optical Components Ltd
Current assignee: Fujitsu Optical Components Ltd
Priority date: 2020-03-05
Filing date: 2021-01-14
Publication date: 2021-09-07
Also published as: US11320595B2; US20210278597A1; JP2021140026A; JP7447559B2

Abstract

Optical devices and optical transceivers including optical modulators. An optical device comprising: a substrate; an optical waveguide forming a Mach-Zehnder interferometer; a signal electrode; and a ground electrode. The optical waveguide is disposed between the signal electrode and the ground electrode. An electric field is generated in a direction along the surface of the substrate when a voltage is applied between the signal electrode and the ground electrode. The optical waveguide includes a first waveguide through which the input light propagates, a curved waveguide optically coupled to the first waveguide, and a second waveguide optically coupled to the curved waveguide. The signal electrode includes first and second electrodes disposed adjacent the first and second waveguides, respectively. An electrical signal is supplied to the first electrode and an anti-phase electrical signal is supplied to the second electrode.

Description

Optical device and optical transceiver including optical modulator

Technical Field

Embodiments discussed herein relate to an optical device including an optical modulator and an optical transceiver.

Background

Fig. 1 shows an example of a conventional optical modulator. In this example, the optical modulator generates a polarization multiplexed optical signal. Thus, the optical modulator includes a pair of parent Mach-Zehnder (Mach-Zehnder) interferometers. Each of the marzehnder interferometers includes a pair of mach-zehnder interferometers.

The optical modulator is formed on an LN substrate (or LN chip). Therefore, the mach-zehnder interferometer is configured by forming an optical waveguide in the LN substrate. In this example, the LN substrate is a Z-cut LN substrate. In this case, the signal electrode is formed above the optical waveguide forming the mach-zehnder interferometer.

In the above-described optical modulator, when an electric signal is applied to the signal electrode, an electric field is generated in a direction perpendicular to the surface of the substrate (i.e., Z direction). These electric fields change the refractive index of the optical waveguide formed below the signal electrode, thereby changing the phase of light. Accordingly, a desired modulated optical signal can be generated by appropriately adjusting the phase of light propagating through the mach-zehnder interferometer using the electrical signal.

In this example, the electrical signal used to drive the optical modulator is a differential electrical signal. The differential electrical signals each include a positive signal and a negative signal having different polarities from each other. In this example, the optical modulator includes an XI modulator, an XQ modulator, a YI modulator, and a YQ modulator. In this case, a pair of electric signals XIp and XIn are supplied to the XI modulator, a pair of electric signals XQp and XQn are supplied to the XQ modulator, a pair of electric signals YIp and YIn are supplied to the YI modulator, and a pair of electric signals YQp and YQn are supplied to the YQ modulator.

Each electrical signal propagates through the corresponding signal electrode and terminates at the RF terminal. In this example, the electrical signals supplied to the signal electrodes each propagate in the same direction at substantially the same speed as the light propagating through the corresponding optical waveguide. In this case, the phase of light propagating through each mach-zehnder interferometer changes in accordance with the electrical signal, and a modulated optical signal is generated.

Further, a known configuration is one in which the voltage of an electrical signal for driving an optical modulator (i.e., a driving voltage) is reduced to reduce the power consumption of the optical modulator. However, reducing the driving voltage involves increasing the length of the region where the light and the electrical signal interact with each other (i.e., the interaction length). Furthermore, increasing the interaction length in the configuration depicted in fig. 1 will extend the length of the LN substrate forming the optical modulator. When doing so in the example depicted in fig. 1, the length of the LN substrate will increase in the lateral direction (or X-direction depicted in fig. 1). As a result, the size of the package for accommodating the optical modulator will increase.

This problem can be solved or alleviated by an optical waveguide having a folded back shape in an LN substrate, as depicted for example in fig. 2. Specifically, in the optical modulator depicted in fig. 2, for both the optical waveguide through which light propagates in the X direction and the optical waveguide through which light propagates in the-X direction, optical and electrical signals interact with each other, thereby significantly extending the interaction length. As a result, the drive voltage can be reduced without extending the length of the LN substrate.

Optical devices having a folded back optical waveguide on a substrate are described in, for example, U.S. 2004/0184755. The technology related to the present application is described in U.S.2008/0226215, japanese patent publication No. 2009-.

As depicted in fig. 1 and 2, in an optical modulator in which a mach-zehnder interferometer is provided in a Z-cut LN substrate, differential electrical signals are supplied to paired optical waveguides forming the mach-zehnder interferometer. For example, the XI modulator depicted in fig. 2 may be driven by a pair of electrical signals XIp and Xin. In this case, it is necessary to accurately adjust the phase and intensity of the pair of electric signals XIp and Xin. However, it would be difficult to design a circuit such that the phases and intensities of a pair of electrical signals arriving at the mach-zehnder interferometer are appropriately adjusted when the rate at which data is transmitted is high. When the phase and intensity of a pair of electrical signals are not properly adjusted, chirp will occur in the modulated optical signal. Therefore, the quality of the transmission signal will deteriorate.

It is an object of one aspect of the present invention to reduce the size of an optical device comprising an optical modulator.

Disclosure of Invention

According to an aspect of the embodiments, an optical device includes: a substrate; an optical waveguide that forms a Mach-Zehnder interferometer in a surface region of a substrate; a signal electrode formed on the substrate; and a ground electrode formed on the substrate. The optical waveguide is disposed between the signal electrode and the ground electrode. The substrate has a property such that an electric field is generated in a direction along the surface of the substrate when a voltage is applied between the signal electrode and the ground electrode. The optical waveguide includes: a first waveguide through which input light propagates in a first direction; a curved waveguide optically coupled to the first waveguide and guiding input light in a second direction different from the first direction; and a second waveguide optically coupled to the curved waveguide. The signal electrodes include a first electrode disposed adjacent the first waveguide and a second electrode disposed adjacent the second waveguide. A first electrical signal of differential electrical signals for driving the mach-zehnder interferometer is supplied to the first electrode, and a second electrical signal of the differential electrical signals is supplied to the second electrode.

Drawings

FIG. 1 illustrates an example of a conventional optical device;

FIG. 2 illustrates another example of a conventional optical device;

FIG. 3 illustrates an example of an optical waveguide forming an optical modulator according to an embodiment of the present invention;

FIG. 4 illustrates an electric field generated by an electrical signal;

FIG. 5 shows an example of an optical modulator formed using an X-cut LN substrate;

FIG. 6 shows an example of an optical device according to an embodiment of the invention;

FIGS. 7A and 7B illustrate electric fields generated in an optical device according to an embodiment of the present invention;

FIG. 8 shows a variation of the optical device depicted in FIG. 6;

FIG. 9 shows another example of an optical device according to an embodiment of the invention;

fig. 10 shows an example of an optical transceiver according to an embodiment of the present invention.

Detailed Description

Fig. 3 shows an example of an optical waveguide forming an optical modulator according to an embodiment of the present invention. In this example, the optical modulator is implemented on a lithium niobate (LN: LiNbO3) substrate 1. Lithium niobate has a high electro-optic coefficient. An optical waveguide having low loss can be formed by diffusing Ti or the like in lithium niobate. Therefore, LN substrates are widely used to realize optical devices such as optical modulators.

As depicted in fig. 3, the optical modulator includes a pair of parent mach-zehnder interferometers MZI _ X and MZI _ Y. Each of the marzehnder interferometers includes a pair of mach-zehnder interferometers. Specifically, the mamch-zehnder interferometer MZI _ X includes a pair of mach-zehnder interferometers XI and XQ. The parent mach-zehnder interferometer MZI _ Y includes a pair of mach-zehnder interferometers YI and YQ.

The mach-zehnder interferometers are formed parallel or substantially parallel to each other. Each mach-zehnder interferometer includes a first waveguide section, a curved waveguide section, and a second waveguide section. The first waveguide section is formed such that input light propagates through the section in a first direction (direction from left to right in fig. 3). The curved waveguide section is optically coupled to the first waveguide section and guides the input light from the first waveguide section in a direction different from the first direction (a direction from right to left in fig. 3). The second waveguide section is optically coupled to the curved waveguide section. In this example, the first and second waveguide sections are formed such that light propagates through the sections in opposite directions to each other. Thus, the curved waveguide section guides light received via the first waveguide section to the second waveguide section such that the light propagates in the opposite direction. Accordingly, the curved waveguide section may be referred to as a "folded back section".

When the optical waveguide is formed in the LN substrate 1 as described above, the input port and the output port are provided on the same edge of the LN substrate 1. In fig. 3, the input port and the output port are provided along the left edge of the LN substrate 1.

The continuous wave light input via the input optical fiber is guided to the marh zehnder interferometers MZI _ X and MZI _ Y. The continuous-wave light in the parent mach-zehnder interferometer MZI _ X is guided to mach-zehnder interferometers XI and XQ. The continuous-wave light in the parent mach-zehnder interferometer MZI _ Y is guided to the mach-zehnder interferometers YI and YQ. The continuous-wave light passing through the mach-zehnder interferometer is modulated by an electrical signal applied to a signal electrode (not shown). As a result, the parent mach-zehnder interferometer MZI _ X generates a modulated optical signal X, and the parent mach-zehnder interferometer MZI _ Y generates a modulated optical signal Y. The modulated optical signals X and Y are combined by a Polarization Beam Combiner (PBC) and directed to an output fiber.

The optical waveguide is formed in the surface region of the LN substrate 1. As an example, as depicted in fig. 4, the optical waveguide may be formed by diffusing Ti or the like in the surface region of the LN substrate 1. It is to be noted that the optical waveguides Ga and Gb depicted in fig. 4 are a pair of optical waveguides forming a mach-zehnder interferometer.

In this example, the LN substrate 1 is an X-cut LN substrate. Therefore, the signal electrode is formed above the region between the pair of optical waveguides forming the mach-zehnder interferometer. In the example depicted in fig. 4, the signal electrode is formed over the area between the optical waveguides Ga and Gb. Further, the ground electrode is formed in other region on the surface of the LN substrate 1. Therefore, the signal electrode and the ground electrode are formed such that the corresponding optical waveguide is disposed therebetween. The buffer layer may be formed on the surface of the LN substrate 1. An insulating layer may be disposed between the buffer layer and the electrode.

When a voltage is applied between the signal electrode and the ground electrode in the optical device, an electric field is generated in the surface region of the LN substrate 1. In this case, since the LN substrate 1 is an X-cut LN substrate, an electric field is generated in a direction along the surface of the LN substrate 1. Accordingly, in the example depicted in fig. 4, an electric field is generated in the + Z direction in the optical waveguide Ga, and an electric field is generated in the-Z direction in the optical waveguide Gb. Therefore, the directions of the electric fields generated in the optical waveguides Ga and Gb are opposite to each other. These electric fields change the refractive indices (or optical path lengths) of the optical waveguides Ga and Gb, respectively, and change the phases of the light output from the optical waveguides, respectively.

Fig. 5 shows an example of an optical modulator formed using an X-cut LN substrate. The optical waveguides formed in the LN substrate 1 depicted in fig. 5 are substantially the same as those depicted in fig. 3. The LN substrate 1 implemented with the optical modulator is housed in a package.

The relay board 2 propagates an electric signal from a driver circuit (not shown) to the LN substrate 1. The electric signals supplied from the driver circuit to the LN substrate 1 include a signal XI, a signal XQ, a signal YI, and a signal YQ. In addition, the ground voltage GND is supplied from the driver circuit to the LN substrate 1 via the relay board 2.

The signal electrode and the ground electrode are formed on the surface of the LN substrate 1. The signal electrodes include an electrode XI through which a signal XI propagates, an electrode XQ through which a signal XQ propagates, an electrode YI through which a signal YI propagates, and an electrode YQ through which a signal YQ propagates. Electrodes XI, XQ, YI, and YQ are formed for Mach-Zehnder interferometers XI, XQ, YI, and YQ, respectively. Specifically, as described above with reference to fig. 4, the electrodes XI, XQ, YI, and YQ are each formed over a region between a pair of optical waveguides forming a corresponding mach-zehnder interferometer. In this example, signal electrodes are provided for the first waveguide section depicted in FIG. 3, and are each formed along an optical waveguide forming a corresponding Mach-Zehnder interferometer. The signal electrode terminates in an RF terminal.

The ground electrode GDN is formed between the signal electrodes. Specifically, the ground electrode GND is formed between each of the pair of electrodes XI and XQ, the pair of electrodes XQ and YI, and the pair of electrodes YI and YQ. The ground electrode is also formed in other regions on the surface of the LN substrate 1, but is not shown in the drawings for clarity.

The continuous wave light is input to the optical modulator via an input optical fiber. Signals XI, XQ, YI, and YQ are supplied from a driver circuit (not shown). Then, the mach-zehnder interferometer MZI _ X modulates the continuous wave light with the signals XI and XQ, thereby generating a modulated optical signal X. The mach-zehnder interferometer MZI _ Y modulates the continuous wave light with the signals YI and YQ, thereby generating a modulated optical signal Y. The modulated optical signals X and Y are combined by a Polarization Beam Combiner (PBC) and directed to an output fiber.

As described above, in the optical modulator formed using the X-cut LN substrate, one signal electrode is formed for each mach-zehnder interferometer, and one electric signal is supplied to the signal electrode. Therefore, it is not necessary to generate a differential electrical signal for driving the mach-zehnder interferometer.

However, in the configuration depicted in fig. 5, the signal electrode is formed only for the first waveguide section depicted in fig. 3. Therefore, the length of the region where light and electric signals interact with each other (i.e., the interaction length) may be small, and thus it is difficult to reduce the driving voltage.

The interaction length may be longer if the signal electrodes are formed for not only the first waveguide section but also the second waveguide section as in the optical modulator depicted in fig. 2. However, when this configuration is applied to the optical modulator depicted in fig. 5, the influence of the electric field generated in the first waveguide section and the influence of the electric field generated in the second waveguide section cancel each other out. For example, assume that, as depicted in fig. 4, in the first waveguide section in a specific mach-zehnder interferometer, an electric field in the + Z direction is generated in the optical waveguide Ga and an electric field in the-Z direction is generated in the optical waveguide Gb. In this case, in the mach-zehnder interferometer, in the second waveguide section, an electric field in the-Z direction is generated in the optical waveguide Ga and an electric field in the + Z direction is generated in the optical waveguide Gb. Thus, the electro-optic effect generated in the first waveguide section and the electro-optic effect generated in the second waveguide section cancel each other out. Therefore, when the LN substrate 1 is an X-cut LN substrate, it is not preferable that the signal electrodes each extend from the first waveguide section to the second waveguide section.

Detailed description of the preferred embodiments

Fig. 6 shows an example of an optical device according to an embodiment of the present invention. In this example, the optical device is an optical modulator that generates a polarization multiplexed optical signal. The LN substrate 1 in fig. 6 is an X-cut LN substrate and is substantially the same as in fig. 5. The optical waveguide formed in the surface region of the LN substrate 1 depicted in fig. 6 is substantially the same as those depicted in fig. 5. Thus, as depicted in fig. 3, the marzehnder interferometers MZI _ X and MZI _ Y are formed in the LN substrate 1. The parent mach-zehnder interferometer MZI _ X includes a pair of mach-zehnder interferometers XI and XQ. The parent mach-zehnder interferometer MZI _ Y includes a pair of mach-zehnder interferometers YI and YQ. Each mach-zehnder interferometer includes a first waveguide section, a curved waveguide section, and a second waveguide section.

The optical modulator depicted in fig. 6 is provided with two signal electrodes for each mach-zehnder interferometer. Specifically, the signal electrodes XIp and Xin are provided for the mach-zehnder interferometer XI, the signal electrodes XQp and XQn are provided for the mach-zehnder interferometer XQ, the signal electrodes YIp and YIn are provided for the mach-zehnder interferometer YI, and the signal electrodes YQp and YQn are provided for the mach-zehnder interferometer YQ. The signal electrode XIp is formed for a first waveguide section of the mach-zehnder interferometer XI and the signal electrode XIn is formed for a second waveguide section of the mach-zehnder interferometer XI. Similarly, the first waveguide section for the Mach-Zehnder interferometer XQ forms signal electrode XQp, and the second waveguide section for the Mach-Zehnder interferometer XQ forms signal electrode XQn. The first waveguide section for the mach-zehnder interferometer YI forms the signal electrode YIp and the second waveguide section for the mach-zehnder interferometer YI forms the signal electrode YIn. The first waveguide section for the mach-zehnder interferometer YQ forms signal electrode YQp, and the second waveguide section for the mach-zehnder interferometer YQ forms signal electrode YQn.

A driver circuit (not shown) generates an electrical signal for driving the mach-zehnder interferometer. In this example, the electrical signal used to drive the mach-zehnder interferometer is a differential electrical signal. Specifically, the driver circuit generates a differential electrical signal XIp/Xin for driving the mach-zehnder interferometer XI, a differential electrical signal XQp/XQn for driving the mach-zehnder interferometer XQ, a differential electrical signal YIp/Yin for driving the mach-zehnder interferometer YI, and a differential electrical signal YQp/YQn for driving the mach-zehnder interferometer YQ. Positive electrical signals XIp, XQp, YIp, and YQp are supplied to electrodes XIp, XQp, YIp, and Yqp, respectively, formed in the first waveguide section. Negative electric signals XIn, XQn, YIn and YQn are supplied to electrodes XIn, XQn, YIn and YQn formed in the second waveguide section, respectively.

The electrical signals propagating through the electrodes XIp, XQp, YIp and YQp are terminated at the RF terminal 3. The electrical signals propagating through electrodes XIn, XQn, YIn and YQn terminate at RF terminal 4.

The ground electrode is formed between the signal electrodes. Specifically, the ground electrode is formed between each of the pair of electrodes XIp and XQp, the pair of electrodes XQp and YIp, and the pair of electrodes YIp and YQp. The ground electrode is also formed between each of the pair of electrodes XIn and XQn, the pair of electrodes XQn and YIn, and the pair of electrodes YIn and YQn. In addition, a ground electrode is also formed between the marh-zehnder interferometers. The ground electrode is also formed in other regions on the surface of the LN substrate 1, but is not shown in the drawings for clarity.

The continuous wave light is input to the optical modulator via an input optical fiber. The continuous wave light is guided to the mach-zehnder interferometers XI, XQ, YI, and YQ. Signals XIp, XQp, YIp, YQp, XIn, XQn, YIn, and YQn are supplied from a driver circuit (not shown). Accordingly, each of the mach-zehnder interferometers XI, XQ, YI, and YQ modulates continuous wave light. The modulation achieved by the mach-zehnder interferometer XI is described below.

The signals XIp and XIn are supplied to the mach-zehnder interferometer XI. Specifically, the signal XIp is supplied to an electrode XIp formed in a first waveguide section of the mach-zehnder interferometer XI. The signal XIn is supplied to an electrode XIn formed in the second waveguide section of the mach-zehnder interferometer XI.

Fig. 7A and 7B illustrate electric fields generated in the optical device according to the embodiment of the present invention. It is to be noted that fig. 7A and 7B depict cross sections of the LN substrate 1 provided when the optical modulator is viewed from the viewpoint V indicated in fig. 6. FIG. 7A depicts a cross-section of a region of a first waveguide section formed with a Mach-Zehnder interferometer XI. FIG. 7B depicts a cross-section of a region of the second waveguide section where the Mach-Zehnder interferometer XI is formed. As depicted in fig. 7A and 7B, the mach-zehnder interferometer XI is formed of a pair of optical waveguides Ga and Gb.

For the first waveguide section, a positive signal XIp is supplied to an electrode XIp formed above the mach-zehnder interferometer XI. The electric field depicted in fig. 7A is generated by a positive signal XIp. Specifically, an electric field propagating in the + Z direction is generated in the optical waveguide Ga, and an electric field propagating in the-Z direction is generated in the optical waveguide Gb.

For the second waveguide section, a negative signal XIn is supplied to an electrode XIn formed above the mach-zehnder interferometer XI. The negative signal XIn is obtained by inverting the positive signal XIp. Thus, an electric field is generated in the second waveguide section that is oriented opposite to the electric field generated by the positive signal XIp in the first waveguide section. Thus, when the electric field depicted in fig. 7A is generated in the first waveguide section, the electric field depicted in fig. 7B is generated in the second waveguide section. Note that the optical waveguide Gb is located on the left side of the optical waveguide Ga in fig. 7A, and on the right side of the optical waveguide Ga in fig. 7B.

As described above, the second waveguide section causes an electric field propagating in the + Z direction to be generated in the optical waveguide Ga and an electric field propagating in the-Z direction to be generated in the optical waveguide Gb, as with the first waveguide section. I.e. the same electro-optical effect is generated in the first waveguide section and the second waveguide section. Thus, the electro-optic effect produced in the second waveguide section by negative signal XIn enhances the electro-optic effect produced in the first waveguide section by positive signal XIp. Thus, an electrode XIp supplied with a positive signal XIp is formed for the first waveguide section, and an electrode XIn supplied with a negative signal XIn obtained by inverting the positive signal XIp is formed for the second waveguide section, thereby significantly increasing the interaction length in the mach-zehnder interferometer XI. The same is true for the Mach-Zehnder interferometers XQ, YI and YQ.

Increasing the interaction length in the mach-zehnder interferometer allows the drive voltage of the signal for driving the optical modulator to be reduced, thereby reducing power consumption. Since each mach-zehnder interferometer includes two waveguide sections disposed parallel to each other and a curved waveguide section coupling the two waveguide sections, the length of the LN substrate 1 can be small. Thus, the configuration depicted in FIG. 6 may reduce the size of an optical modulator with low power consumption.

In the configuration depicted in fig. 6, the electrical signal used to drive the optical modulator is a differential electrical signal. In this case, a pair of electric signals forming a differential electric signal are supplied to electrodes independent of each other, and each act on a different region of the optical waveguide. Specifically, one of the electrical signals (e.g., signal XIp) changes the refractive index of the first waveguide segment, and the other electrical signal (e.g., signal XIn) changes the refractive index of the second waveguide segment. Therefore, it is not necessary to accurately adjust the phase and intensity of a pair of electric signals forming a differential electric signal. Thus, the optical modulator depicted in FIG. 6 may have a simplified design to supply electrical signals to the Mach-Zehnder interferometer.

However, the modulation operation on the first waveguide section and the modulation operation on the second waveguide section need to be synchronized with each other. The input light passes through the first waveguide section and is then guided to the second waveguide section via the curved waveguide section. Thus, the modulation operation on the first waveguide section is performed first, and then the modulation operation on the second waveguide section is performed. Thus, each signal electrode is configured such that the electrode through which the electrical signal propagates from the driver circuit to the second waveguide section is longer than the electrode through which the electrical signal propagates from the driver circuit to the first waveguide section. For example, the conductor pattern (or wiring pattern) used to propagate the electrical signal may be designed for the mach-zehnder interferometer X1 such that the difference between the time required for the signal XIp to propagate from the driver circuit to point P1 and the time required for the signal XIn to propagate from the driver circuit to point P2 substantially matches the time required for the light to propagate from point P1 to point P2 via the optical waveguide. It is to be noted that the conductor pattern for propagating the electrical signal from the driver circuit to the second waveguide section includes a lead between the edge of the package and the edge portion of the LN substrate 1, and the lead is formed on, for example, a relay board (not shown).

As described above, in the configuration depicted in fig. 6, the conductor pattern for propagating an electrical signal to the second waveguide section is longer than the conductor pattern for propagating an electrical signal to the first waveguide section. In this regard, when the rate of the data signal transmitted through the optical modulator is high, the longer the electrode, the greater the loss in the transmitted signal will be. Thus, consider that in the configuration depicted in fig. 6, the quality of the electrical signal supplied to the second waveguide section is lower than the quality of the electrical signal supplied to the first waveguide section.

Thus, the signal electrodes for each mach-zehnder interferometer in the configuration depicted in fig. 8 are formed such that the interaction length of the second waveguide section is smaller than the interaction length of the first waveguide section. In fig. 8, the electrodes XIp, XQp, YIp, and YQp and the electrodes XIn, XQn, XIn, and YQn are formed such that the interaction length L2 is smaller than the interaction length L1. With this configuration, modulation using a high-quality electrical signal becomes dominant, thereby increasing the quality of the modulated optical signal generated by the optical modulator.

Fig. 9 shows another example of an optical device according to an embodiment of the present invention. The optical device depicted in fig. 9 includes a modulator chip 10, a driver circuit 20, a relay board 31, and a relay board 33. The modulator chip 10, the driver circuit 20, the relay board 31, and the relay board 33 are accommodated in a package. The input optical fiber and the output optical fiber are connected to the package. The relay board 31 is a laminated board (or a multilayer board).

The modulator chip 10 is configured using an X-cut LN substrate. An optical modulator that generates a polarization multiplexed optical signal is implemented on a modulator chip 10. The optical waveguides in the surface region of the LN substrate form the Mach-Zehnder interferometers MZI _ X and MZI _ Y, as in the configurations depicted in FIGS. 6 and 8. The parent mach-zehnder interferometer MZI _ X includes a pair of mach-zehnder interferometers XI and XQ. The parent mach-zehnder interferometer MZI _ Y includes a pair of mach-zehnder interferometers YI and YQ. In this example, each of the mamachy-zehnder interferometers XMI _ X and MZI _ Y generates a QPSK optical signal.

A DC bias electrode is provided for each of the mach-zehnder interferometers XI, XQ, YI, and YQ. DC bias voltages for adjusting the operating points of the Mach-Zehnder interferometers XI, XQ, YI, and YQ are applied to the DC bias electrodes. The DC bias voltage is generated by a control circuit (not shown).

A phase adjustment bias electrode is provided for each of the MechZehnder interferometers MZI _ X and MZI _ Y. DC bias voltages each for adjusting a phase difference between a pair of Mach-Zehnder interferometers among the Mach-Zehnder interferometers are applied to the phase adjustment bias electrodes. The DC bias voltage is generated by a control circuit (not shown).

The driver circuit 20 generates an electrical signal for driving the optical modulator. Specifically, the driver circuit 20 generates electrical signals for driving the mach-zehnder interferometers XI, XQ, YI, and YQ. The electrical signal is a differential electrical signal. Specifically, the differential electrical signal supplied to the mach-zehnder interferometer XI is formed by a positive signal XIp and a negative signal XIn. The differential electrical signal supplied to the mach-zehnder interferometer XQ is formed from a positive signal XQp and a negative signal XQn. The differential electrical signal supplied to the mach-zehnder interferometer YI is formed from a positive signal YIp and a negative signal Yin. The differential electrical signal supplied to the mach-zehnder interferometer YQ is formed from a positive signal YQp and a negative signal YQn.

The positive signal (XIp, XQp, YIp, YQp) to be supplied to the first waveguide section among the electric signals generated by the driver circuit 20 propagates to the pad formed near the modulator chip 10 through the wire formed on the surface of the relay board 31. A blocking capacitor for cutting the DC component is provided on the lead. The blocking capacitor is denoted as "C" in fig. 9. The leads are each electrically connected to a corresponding electrode formed on the modulator chip 10.

Negative signals (XIn, XQn, YIn, YQn) to be supplied to the second waveguide section among the electric signals generated by the driver circuit 20 propagate to pads formed near the relay board 33 through wires formed in the inner layer of the relay board 31. The lead lines formed in the inner layer are indicated by broken lines in fig. 9. The leads formed on the surface and the leads formed in the inner layer are electrically connected to each other. The through holes are indicated by double circles in fig. 9. In addition, a blocking capacitor is also provided on these leads. These leads are each electrically connected to a corresponding wiring pattern formed on the relay board 33.

A wiring pattern for propagating a negative signal to be supplied to the second waveguide section is formed on the relay board 33. These wiring patterns are each electrically connected to a corresponding electrode formed on the modulator chip 10.

In the modulator chip 10, positive signals XIp, XQp, YIp, and YQp are supplied to the first waveguide sections of the respective mach-zehnder interferometers XIp, XQp, YIp, and YQp. These positive signals are terminated at an RF terminal 34 provided on the relay board 33.

In the modulator chip 10, negative signals XIN, XQn, YIn, and YQn are supplied to the second waveguide sections of the corresponding Mach-Zehnder interferometers XIN, XQn, YIn, and YQn. These negative electrode signals are terminated at RF terminals 32 provided on the relay board 31.

In the optical device, pairs of positive and negative signals forming a differential electrical signal are output from the driver circuit 20 via terminals disposed close to each other. Further, the plurality of positive signals (XIp, XQp, YIp, YQp) and the plurality of negative signals (XIn, XQn, YIn, YQn) respectively need to be concentrated when supplied to the modulator chip 10. Therefore, the path of the lead for propagating the positive signal and the path of the lead for propagating the negative signal overlap each other.

Therefore, in this example, a laminate is used as the relay board 31. Portions of the leads for propagating negative signals are formed in inner layers in the laminate. Therefore, the negative signal propagates through the lead formed on the surface of the relay board 31 and the lead formed in the inner layer in the relay board 31.

As described above, the longer the lead from the driver circuit 20 to the mach-zehnder interferometer, the lower the quality of the electrical signal will be. Therefore, the wiring pattern is designed so that at least a positive signal or a negative signal propagates from the driver circuit 20 to the mach-zehnder interferometer by taking a path as short as possible. In this example, the wiring pattern is designed such that the positive signal propagates from the driver circuit 20 to the mach-zehnder interferometer by taking as short a path as possible. Therefore, in the relay board 31, the lead of the negative signal propagation is longer than the lead of the positive signal propagation.

In many cases, the loss in the lead lines in/on the laminate is larger than the loss in the lead lines formed on the single-layer board. Therefore, when the portion of the lead between the driver circuit 20 and the mach-zehnder interferometer is formed in/on the laminate (i.e., relay board 31), the lead formed in/on the laminate is preferably as short as possible. Thus, in this example, the relay board 33 is a single-layer board. When both the laminate (i.e., the relay board 31) and the single-layer board (i.e., the relay board 33) are provided with the lead wires for propagating the negative signal, the lead wires formed in/on the laminate will preferably be short, and the lead wires formed on the single-layer board will preferably be long.

In addition, the optical modulator depicted in fig. 9 includes four mach-zehnder interferometers XI, XQ, YI, and YQ, and in this regard, it is necessary to appropriately adjust skew of electrical signals for driving the four mach-zehnder interferometers. In particular, the length of time it takes for the electrical signal to travel from the driver circuit 20 to the mach-zehnder interferometer needs to be the same or substantially the same. In this case, for example, the signal electrode on the LN substrate may be formed so as to minimize the length of a path through which the electric signal propagates. When doing so, the lengths of the signal electrodes formed on the LN substrate may be different. In this case, skew is adjusted by appropriately adjusting the length of the lead wire formed on the relay board.

Fig. 10 shows an example of an optical transceiver according to an embodiment of the present invention. Optical transceiver 100 includes an optical source (LD)101, an optical modulator 102, an optical receiver 103, and a Digital Signal Processor (DSP) 104.

For example, the light source 101 is a laser light source and generates continuous wave light of a specified frequency. The optical modulator 102 generates a modulated optical signal by modulating the continuous wave light generated by the light source 101 with a transmission signal supplied from the DSP. For example, the optical modulator 102 may correspond to the optical device depicted in fig. 6, 8, or 9. For example, the optical receiver 103 is a coherent receiver, and demodulates a received optical signal using continuous wave light generated by the light source 101. The DSP 104 generates a transmission signal from data supplied from an application. The transmission signal is supplied to the optical modulator 102. The DSP 104 also recovers data from the received signal demodulated by the optical receiver 103.

Accordingly, the optical transceiver 100 includes the optical device according to the embodiment of the present invention as an optical modulator. Therefore, both reduction in power consumption of the optical transceiver and size reduction of the optical transceiver can be achieved.

Claims

1. An optical device, comprising:

a substrate;

an optical waveguide that forms a Mach-Zehnder interferometer in a surface region of the substrate;

a signal electrode formed on the substrate; and

a ground electrode formed on the substrate, wherein

The optical waveguide is disposed between the signal electrode and the ground electrode,

the substrate has a property such that an electric field is generated in a direction along a surface of the substrate when a voltage is applied between the signal electrode and the ground electrode,

the optical waveguide includes:

a first waveguide through which input light propagates in a first direction,

a curved waveguide optically coupled to the first waveguide and guiding the input light in a second direction different from the first direction, an

A second waveguide optically coupled to the curved waveguide,

the signal electrode includes:

a first electrode disposed adjacent to the first waveguide, an

A second electrode disposed adjacent to the second waveguide and

a first electrical signal of differential electrical signals for driving the mach-zehnder interferometer is supplied to the first electrode, and a second electrical signal of the differential electrical signals is supplied to the second electrode.

2. The optical device of claim 1,

a wiring pattern between a driver circuit for generating the differential electric signal and the second electrode is longer than a wiring pattern between the driver circuit and the first electrode.

3. The optical device of claim 1,

an area where the second waveguide and the second electrode are parallel to each other is shorter than an area where the first waveguide and the first electrode are parallel to each other.

4. The optical device of claim 1, further comprising:

one or more relay boards between the driver circuit for generating the differential electrical signals and the substrate, wherein,

at least one of the one or more relay boards is a laminate board.

5. The optical device of claim 4,

in the laminate, a first wiring pattern provided between the driver circuit and the first electrode is formed only on a surface of the laminate, and

in the laminate, a part of a second wiring pattern provided between the driver circuit and the second electrode is formed in an inner layer in the laminate.

6. The optical device of claim 5,

the first wiring pattern is shorter than the second wiring pattern.

7. An optical transceiver comprising an optical modulator and an optical receiver, wherein,

the optical modulator includes:

a substrate, a first electrode and a second electrode,

an optical waveguide of a Mach-Zehnder interferometer is formed in a surface region of the substrate,

a signal electrode formed on the substrate, and

a ground electrode formed on the substrate, wherein,

the optical waveguide includes:

a first waveguide through which input light propagates in a first direction,

A second waveguide optically coupled to the curved waveguide,

the signal electrode includes:

a first electrode disposed adjacent to the first waveguide, an

A second electrode disposed adjacent to the second waveguide, and